1. Field of the Invention
The present invention relates to semiconductor integrated circuits, and more particularly to Large Scale Integration (LSI) circuits comprising a plurality of mutually connected unit cells on a single semiconductor wafer.
A semiconductor integrated circuit consists of functional circuits integrally formed on a single semiconductor substrate, so that circuit means using integrated circuits are characterized by their miniaturized dimensions and a remarkably high reliability in operation. Accordingly, the semiconductor integrated circuits are extremely valuable in the field of electronic computers, the main subject of which is to process information with accuracy as well as at a high speed, and thus most of the logic circuits or the like in use are in the form of an integrated circuit. In the field of semiconcuctor integrated circuits, efforts have been made to increase the degree of integration of circuit components to be formed on a single semiconductor substrate with the progress of the integrated circuit manufacturing technique, so that the function of a larger-scale circuit may be performed by a single integrated circuit. In the present specification, the term "unit cell" is used to involve a fundamental functional circuit which is normally used to construct any various system. For example, the unit cell may be an AND circuit, a NAND circuit, an OR circuit, a Flip-Flop circuit or the like, while the term "LSI circuit" is used to involve a circuit consisting of either a plurality of integrated unit cells formed on a single semiconductor substrate or a much greater number of integrated circuit components as compared with a unit cell.
2. DESCRIPTION OF THE PRIOR ART
Conceptually, the LSI circuit is a development of the conventional Integrated Circuit (IC). However, practically, since in the LSI circuit an extremely great number of circuit components must be contained on one substrate, unexpected complication which has not been experienced in the conventional manufacture of integrated circuits arises in carrying out the circuit design and circuit component location for forming an LSI circuit. Therefore, the custom approach to a maximum utility of the area of a substrate by individually designing the component location and metallization connection for each required circuit has been effective in the IC field, but is not practical in the LSI field where the circuit arrangement is much more complicated. One of the strongest reasons why the custom approach is not suited for the LSI is that the design of LSI circuits is intricate resulting in high manufacturing costs since the custom approach necessitates designing, for each individual LSI circuit, both an impurity diffusion mask for forming circuit elements and an interconnection mask for interconnecting the circuit elements.
In order to remove such deficiency present in the custom approach, there have been proposed various approaches for the LSI, and the master slice approach can be given here as example. In the master slice approach, a great number of circuit elements are formed and arranged on a substrate beforehand so as to be able to obtain different LSI circuits by modifying the interconnecting metallization, so that any desired circuits may be completed by suitably connecting the circuit elements having been formed and arranged as described. This approach is, therefore, advantageous in that the same diffusion mask can be used to produce a plurality of kinds of LSI circuits merely by designing masks for effecting metallization, which is the final fabrication step, required for individual LSI circuits. What is most important for the master slice approach is which kind of circuit elements are involved and how they should be arranged. This factor determines the flexibility of an LSI circuit. In view of this fact a proposal was made, which is known from U.S. Pat. No. 3,365,707 granted to Thomas R. Mayhew et al. According to the LSI circuit proposed by T. R. Mayhew, each unit cell is designed to have such a structure that it includes four insulated gate field-effect devices having both committed and uncommitted connections, whereby a system designer is given the flexibility of specifying the functional identity of a cell by means of the design connection pattern of the various uncommitted connecting points. In the LSI circuit by Mayhew et al, though unit cells have the same structure, they can be modified so as to have different functional identities depending upon the way of interconnecting the same, and therefore high flexiblity or adaptability is provided for designing any LSI circuit.
However, since the LSI structure by Mayhew et al is in accordance with the master slice system, extra elments which are unnecessary for a desired LSI circuit construction are necessarily formed on a semiconductor substrate. As a result, the utility of the substrate area is disadvantageously poor. Furthermore, this structure is defective in that the number of and the number of kinds of circuit elements capable of being included in each unit cell are limited so that a desired LSI circuit can not be obtained, or if the latter is obtained the wiring design would be too complicated, and many useless elements tend to be formed additionally.